This invention relates to a method for controlling an ultrasonic hyperthermia treatment apparatus having a transducer with a plurality of piezoelectric elements each excitedly an electrical signal, in which ultrasound is focused at a focal spot and the position of said focal spot is varied with respect to the transducer at a speed that is higher than the speed at which thermal energy diffuses away from the area being treated.
The invention also provides a hyperthermia treatment apparatus including a transducer with a plurality of piezoelectric elements excited by a plurality of electrical excitation signals, with means for variably phasing each one of said electrical excitation signals.
The fact that ultrasonic waves, referred to generally as ultrasound, can be propagated within the human body and also can be easily focused is used in medicine for acting on internal structures from an external point in non-invasive therapy without the need for surgical intervention. The ultrasonic waves are focused into a specific region referred to as the focal point or focal spot. For the purposes of treatment, the focal point is brought to the region to be treated.
Hyperthermia treatment with focusing of ultrasound leads to local heating of tissue at the focal point with the consequent destruction thereof.
Known apparatus for carrying out hyperthermia treatment is described, for example in European Patents EP-A-0 162 735 (semi-spherical cup reflector and fixed focal spot of some 2 or 3 mm diameter), EP-A-0 370 841 (mechanically movable transducer), or EP-A 0 194 897 (scanning of treatment zone by mechanically shifting the transducer). A more complete description of these various devices and their disadvantages is given in French patent application No. 94.00904 (priority document for this present application), the contents of which are incorporated herein by reference.
An article by Ebbeni et al entitled "A spherical section ultrasound phased array for deep localisation hyperthermia", IEEE Transactions on Biomedical Engineering, vol. 38, no. 7, Jan. 7, 1991, pages 634-643 describes the results of computer simulation of transmission of ultrasound by a spherical-section array incorporating a plurality of piezoelectric elements. In this article, it is suggested to move the focal spot by varying the amplitude and phase of the various piezoelectric elements in order to irradiate a plurality of control points arranged on a circle, or on two concentric circles. It is however considered as preferable in this document to directly generate an ultrasound field incorporating the plurality of control point.
U.S. Pat. No. 4,938,217 discloses a hyperthermia device comprising a transducer having a plurality of piezoelectric elements which combines mechanical movement of the transducer and modification of the depth of the focal spot or its distance from the transducer by appropriately phasing the excitation signals, this being referred to as apodization. In this patent, varying the focal length during rotation of the transducer enables irregularly-shaped regions to be scanned.
These various known devices have certain disadvantages.
It is difficult to synthesise focal spots of large size as in EP-A-0,194,897, or which exhibit a plurality of peaks as in the Ebbeni et al article: firstly, this implies complex phase and amplitude calculations for the excitation signals and, secondly, in the case of actual treatment apparatus having a reasonable number of piezoelectric elements, this creates irregularities around the focal spot which can cause lesions.
If, as opposed to this, focal spots of reduced dimension are employed, it becomes difficult to treat large volumes. Mechanical scanning of the treatment zone, through movement of the transducer (EP-A-0,194,897 or U.S. Pat. 4,938,217) requires the presence of mechanical means for moving the transducer, which leads to cumbersome and complicated equipment. Moreover, numerous shots are necessary and if the region requiring treatment is itself moving due to the patient's breathing, the points of impact are distributed in a somewhat random fashion within the volume theoretically scanned. The treatment region cannot be fully scanned (presence of gaps), and some points of impact can even fall outside this region.
Electronically steering the focal spot using apodization, as suggested in the article by Ebbeni et al or in U.S. Pat. No. 4,938,217 is a technique that is complicated to implement: it is in fact difficult to modify the peak power of the ultrasonic waves delivered by each individual piezoelectric element particularly when very rapid changes are needed.
One further problem involves the question of how to distribute the energy within the focal spot. In the article by Ebbeni et al, it is proposed, in order to determine the energy necessary to attain and maintain a certain level of hyperthermia, to calculate a spatio-temporal mean within the volume of the tumour of the energy deposited at the control points. This type of approach can only be usefully applied in the case of so-called "conventional" hyperthermia, i.e. for large volumes and moderate temperatures (43.degree. to 45.degree. C.). U.S. Pat. No. 4,938,217 simply states that it is desirable to obtain a uniform temperature within the tumour, and to avoid excessive overheating. There is no indication in this document of how to choose the energy level at the focal spot or at the various points within the region to be treated. In known devices, losses through diffusion of heat energy represent a significant portion of the energy delivered to the patient and a high percentage of the energy radiated is not used for cell destruction. This is problematic as an excess of energy radiated can lead to lesions as a result of burning, either of internal tissue or of the skin.